Conventionally, the number of vehicles including, as a headlight, an HID (High Intensity Discharged) lamp instead of a halogen lamp had been increased, in order to improve the visibility (brightness). However, recently, due to the improvement of luminous efficiency of an LED (light emitting diode), mass production of vehicles including, as the headlight, the LED has been already started. For example, JP2011-113643A (hereinafter, referred to as “document 1”) discloses a lighting device, which is configured to light a headlight that is an LED.
The lighting device in the document 1 includes a converter, a voltage detector, a current detector, a controller and a temperature detector. The converter is configured to convert a DC voltage, received from a DC power supply, into a voltage at which a load (LED) can be lighted. The voltage detector is configured to detect an output voltage of the converter, or a value corresponding to the output voltage. The current detector is configured to detect an output current of the converter, or a value corresponding to the output current. The controller is configured to control the converter based on a value detected by the voltage detector and/or the current detector. The temperature detector is configured to detect the temperature of the lighting device, or a value corresponding to the temperature.
When the temperature detected by the temperature detector becomes higher than a predetermined temperature, the controller is configured to reduce the output current according to the elapsed time. Therefore, this lighting device can suppress an increase of a circuit loss at a high temperature, and prevent a failure in the device.
However, if the output current to be supplied to the load is reduced as the above conventional example, luminous flux of the load is decreased, and accordingly, there is a possibility that sufficient illumination cannot be provided. For example, in a case where the load is a headlight, the luminous flux of the load upon lighting of a passing light (so-called low-beam) is lower than that upon lighting of a running light (so-called high-beam). In addition, a lighting distribution area upon lighting of the low-beam is more strictly limited depending on the standard, compared with that upon lighting of the high-beam. Therefore, if the control of reducing the output current is performed upon lighting of the low-beam, illumination of the low-beam cannot be provided sufficiently, and accordingly, there is a possibility that visibility of a driver is reduced.